1. Field of the Invention
The present invention relates to a small filter suitable for radio communication equipments such as cellular phones.
2. Background Art
Size reduction is required for a filter used for radio communication equipments such as cellular phones. Therefore, size reduction is required even for resonators configuring the filter. A filter has been developed in the past, in which each resonator is configured using a TEM (Transverse Electro Magnetic) line to achieve size reduction. Here, as a method of coupling two resonators including the TEM line respectively to each other, two types of coupling can be typically listed: comb-line coupling and interdigital coupling. Japanese Patent No. 3067612 and Japanese Unexamined Patent Publication No. 2007-180684 disclose inventions in which a small band-pass filter is configured using interdigital-coupled resonators, respectively.